Horizon Zero Dawn's The War-Chief's Trial unlocks as part of A Seeker At The Gates, and is a mission we recommend completing at your earliest opportunity.
If you're after help for other parts of the game, consult our complete Horizon Zero Dawn walkthrough and guide.
Go to the Massacre Site and go to Sona's Last Known Location
Speaking with Varl reveals that he's been stuck here holding the gate since a recent massacre of their War Party took out most of their troops. Their War-Chief, Sona, is missing - and that chap Resh is useless - and Varl can't hunt her down himself as she commanded him to hold the gate.
It's a bit of a mess, basically, and it's down to you to go clear things up. Get as much detail as you can out of Varl, and then get to work hunting down the Nora's War-Chief, starting at the site of the massacre.
Go to the Massacre Site - Carry on with your mount to the waypoint marked on your map as the site of Sona's troops' massacre.
Investigate the Battlefield (Examine the Destroyed Corruptor) - Use your Focus to investigate what happened, scanning the destroyed Corruptor machine as you go.
Talk to the Survivor - A survivor appears, and has some information to share. Go to Sona's Last Known Location - He knows here last location, at least, before disappearing. Maybe there's something there for you to discover.
Investigate the Area (Examine the Campfire/Bow/Arrowheads/Bandages) - Up at her last known location, you'll find a selection of scannable items on the floor. Scan and then examine all of them using your Focus, before discovering a trail.
Follow the Tracks - Highlighting the tracks with your Focus, follow them to see where they lead.
Examine the Sawtooth - A dead Sawtooth lines the way, see if it has anything to reveal.
Follow Sona - At last, you find Sona. She's been tracking the killers from the Proving ambush to a camp nearby. Follow her to get some vengeance.
Kill the Cultists and Corrupted Machines (Destroy the Blaze Containers [Optional]) - Wipe out all the enemies in site using whatever means you prefer. It's possible to do this entirely stealthily or going in guns (and Blaze Containers) blazing.
Talk to Sona - With the enemies defeated, catch up with Sona after the battle.
Investigate the Dig Site (follow the Trail of Blaze/Listen to the Datapoint) - use your focus to find a trail of Blaze, and a Datapoint. Investigate, then talk to Sona and Varl again. Talk to Sona and Varl - There's more revenge to be had just yet, it seems, so on to your next quest!
For more Horizon guides, explainers and advice, take a look at our main Horizon Zero Dawn guide and walkthrough hub, or our advice on finding Horizon Zero Dawn's override Cauldrons for controlling any machine, and unlocking some super-strong armour with our Horizon Zero Dawn Power Cell locations guide, too.
With that, you've finished The War-Chief's Trail quest, and now have a choice: head back to the original branch of the story, by continuing A Seeker at the Gates, or carrying on down your current path and helping the Nora seek vengeance in Revenge of the Nora.
We recommend the latter while you're here, as it gives you some useful story context and experience before you return to the main storyline.